In general, a substrate may be processed in a lithographic projection apparatus comprising: a radiation system to supply a projection beam of radiation; a support structure to support patterning structure, the patterning structure serves to pattern the projection beam according to a desired pattern; a substrate table to hold a substrate; and a projection system to project the patterned beam onto a target portion of the substrate.
The term “patterning structure” as here employed should be broadly interpreted as referring to structure or means that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate; the term “light valve” can also be used in this context. Generally, the said pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). Examples of such patterning structure include:                A mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support structure will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired.        A programmable mirror array. One example of such a device is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that (for example) addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the said undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. An alternative embodiment of a programmable mirror array employs a matrix arrangement of tiny mirrors, each of which can be individually tilted about an axis by applying a suitable localized electric field, or by employing piezoelectric actuation means. Once again, the mirrors are matrix-addressable, such that addressed mirrors will reflect an incoming radiation beam in a different direction to unaddressed mirrors; in this manner, the reflected beam is patterned according to the addressing pattern of the matrix-addressable mirrors. The required matrix addressing can be performed using suitable electronic means. In both of the situations described hereabove, the patterning structure can comprise one or more programmable mirror arrays. More information on mirror arrays as here referred to can be gleaned, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, and PCT patent applications WO 98/38597 and WO 98/33096, which are incorporated herein by reference. In the case of a programmable mirror array, the said support structure may be embodied as a frame or table, for example, which may be fixed or movable as required.        A programmable LCD array. An example of such a construction is given in U. S. Pat. No. 5,229,872, which is incorporated herein by reference. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table; however, the general principles discussed in such instances should be seen in the broader context of the patterning structure as hereabove set forth.        
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning structure may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at one time; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatus —commonly referred to as a step-and-scan apparatus—each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the “scanning” direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally <1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
In a manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book “Microchip Fabrication: A Practical Guide to Semiconductor Processing”, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4, incorporated herein by reference.
For the sake of simplicity, the projection system may hereinafter be referred to as the “lens”; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens”. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such “multiple stage” devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Dual stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and PCT Patent Application WO 98/40791, incorporated herein by reference.
This invention relates to an alignment mark for use in an alignment system of a lithographic projection apparatus for aligning the pattern in the patterning structure to the substrate. This alignment system, for example, the indirect off-axis alignment system described in PCT Patent Application WO 98/39689 incorporated herein by reference, employs an alignment beam of radiation that is radiated by a separate alignment unit and that is incident on a mark, in the form of a grating on the substrate. The grating diffracts the alignment beam into a number of sub-beams extending at different angles to the normal of the grating. Said distinct sub-beams will be directed with a lens of the alignment unit to different positions in a plane. In this plane means may be provided for further separating the different sub-beams. The lens system will also be used to finally image the different sub-beams on a reference plate to create an image of the mark. In this reference plate a reference mark can be provided and a radiation sensitive detector can be arranged behind the reference mark. The output signal of the detector will be dependent on the extent to which the image of the substrate mark and the reference mark coincide. In this way the extent of alignment of the mark on the substrate with the reference mark in the alignment unit can be measured and optimized. The detector may comprise separate individual detectors for measuring the intensity and the aligned position at different orders. To finish the alignment, the reference in the alignment unit has to be aligned to a second reference mark, for example, one provided to the substrate table with the alignment unit. This second reference mark may then be aligned to a mark in the mask using exposure light. An apparatus and method as described in U.S. Pat. No. 5,144,363, incorporated herein by reference can be used for that purpose.
Alternatively, a direct on-axis alignment system can be used that directs an alignment beam directly upon a mark provided on the substrate via the projection system. This beam will be diffracted by the mark on the substrate into different sub-beams and will be reflected into the projection system. After traversing the projection system the different sub-beams will be focussed on a reference alignment mark provided to the mask. The image of the substrate mark formed by the sub-beams can be imaged upon the reference mark in the mask. In this way the extent of alignment of the mark on the substrate and the reference mark in the mask can be measured and optimized. This can be done by using a radiation sensitive detector constructed and arranged to detect the alignment beam traversing the mark in the mask. For more information with respect to the on-axis alignment system here described see, for example, U.S. Pat. No. 4,778,275 incorporated herein by reference.
To improve the speed of integrated circuits produced with the lithographic projection apparatus, it is proposed to use low resistance materials, such as copper, as a conductor to decrease time delays in the circuits. To fabricate integrated circuits with copper, a new substrate processing technique known as copper dual damascene is introduced in the manufacturing of integrated circuits. One of the problems that occurs with this new processing technique is that during alignment to marks embedded in a copper dual damascene layer a weak or even no signal is measured by the alignment system. It is discovered that the radiation of the alignment beam traversing through the mark and the copper dual damascene layer to the substrate surface may reflect thereupon and may traverse back to the alignment mark where it may interfere with radiation of the alignment beam directly reflected upon the mark. The problem is caused by the layer of copper dual damascene being substantially transmissive for the alignment beam of the alignment system. The interference is largely dependent on the thickness of the copper dual damascene layer on which the mark is laying. It is however very difficult to control the thickness of the layer to such an extent that the interference can be avoided.